13.
a)$\left \{ {{3x-y=1} \atop {6x-2y=5}} \right.$
=>$\left \{ {{6x-2y=2} \atop {6x-2y=5}} \right.$
=>$\left \{ {{0x-0y=2(vô lí)} \atop {6x-2y=5}} \right.$
=> hpt vô nghiệm
b)$\left \{ {{2x+y=11} \atop {5x-4y=8}} \right.$
=>$\left \{ {{8x+4y=44} \atop {5x-4y=8}} \right.$
=>$\left \{ {{13x=52} \atop {5x-4y=8}} \right.$
=>$\left \{ {{x=4} \atop {y=3}} \right.$
c)$\left \{ {{3x-2y=7} \atop {2x+3y=2}} \right.$
=>$\left \{ {{6x-4y=14} \atop {6x+9y=6}} \right.$
=>$\left \{ {{-13y=8} \atop {2x+3y=2}} \right.$
=>$\left \{ {{y=\frac{-8}{13}} \atop {x=\frac{25}{13}}} \right.$
d)$\left \{ {{4x+7y=16} \atop {4x-3y=-24}} \right.$
=>$\left \{ {{10y=40} \atop {4x-3y=-24}} \right.$
=>$\left \{ {{y=4} \atop {x=-3}} \right.$
15.
a)$\left \{ {{5x+2y=2} \atop {x+\frac{2}{5}y=1}} \right.$
=>$\left \{ {{5. (1-\frac{2}{5}y)+2y=2} \atop {x=1-\frac{2}{5}y }} \right.$
=>$\left \{ {{5-2y+2y=2} \atop {x=1-\frac{2}{5} }} \right.$
=>$\left \{ {{5-0y=2(vô lí)} \atop {x=1-\frac{2}{5}}} \right.$
=> pt vô nghiệm
b)$\left \{ {{10x-9y=8} \atop {15x+21y=0,5}} \right.$
=>$\left \{ {{x=\frac{8+9y}{10} } \atop {15.\frac{8+9y}{10}+21y=0,5}} \right.$
=>$\left \{ {{x=\frac{8+9y}{10}} \atop {24+27y+42y=1}} \right.$
=>$\left \{ {{x=\frac{8+9y}{10} } \atop {69y=-23}} \right.$ =>$\left \{ {{x=\frac{1}{2} } \atop {y=\frac{-1}{3}}} \right.$
c)
$\left \{ {{2x+y=11} \atop {5x-4y=8}} \right.$
=>$\left \{ {{y=11-2x} \atop {5x-4(11-2x)=8}} \right.$
=>$\left \{ {{y=11-2x} \atop {5x-44+8x=8}} \right.$
=>$\left \{ {{y=11-2x} \atop {13x=52}} \right.$
=>$\left \{ {{y=3} \atop {x=4}} \right.$
d)
$\left \{ {{2x+y=1} \atop {2x+4y=-1}} \right.$
=>$\left \{ {{y=1-2x} \atop {2x+4(1-2x)=-1}} \right.$
=>$\left \{ {{y=1-2x} \atop {2x+4-8x=-1}} \right.$
=>$\left \{ {{y=1-2x} \atop {-6x=-5}} \right.$
=>$\left \{ {{y=\frac{-2}{3} } \atop {x=\frac{5}{6} }} \right.$
